This invention relates to marine propulsion devices such as outboard motors, and more particularly to water jacketed exhaust discharge systems including an exhaust gas relief arrangement.
Exhaust gas from outboard motor internal combustion engines typically is directed downwardly through a gas expansion chamber in a drive shaft housing and then discharged into the water through a through-the-hub propeller or the like. The exhaust gas expansion chamber is jacketed with water to cool the exhaust gases and muffle the sound.
At higher boat speeds, a low pressure region is created behind the propeller and exhaust gases are easily discharged into the water. At engine idle or lower boat speeds, water backs up through the hub into the exhaust gas expansion chamber and creates a static back pressure which restricts the discharge of exhaust gases and creates rough engine operating characteristics if those exhaust gases are not allowed escape through an alternative means.
Exhaust relief systems, or secondary exhaust discharge means have been provided for venting the exhaust gases to atmosphere during engine idle and low boat speeds through a discharge outlet located in the drive shaft housing or lower unit generally above the water line when the boat and outboard motor is at rest or at low speeds. These secondary exhaust discharge systems also generally allow at least some exhaust to be vented during high speed operation. Examples of prior art constructions including exhaust relief systems are disclosed in the following U.S. patents:
______________________________________ PATENT NUMBER INVENTOR(S) ISSUE DATE ______________________________________ 3,967,446 Harralson, J. H. 7/6/76 Wurzbach, W. F. 4,145,988 Harada, Norimichi 3/27/79 4,303,401 Sanmi, Yasumaru 12/1/81 Harada, Norimichi Nishida, Yasuhiro 4,354,849 Sanmi, Yasumaru 10/19/82 Harada, Norimichi Nishida, Yasuhiro 4,421,490 Nakahama, Ryoji 12/20/83 4,604,069 Taguchi, Michihiro 8/5/86 4,607,723 Okazaki, Masaki 8/26/86 4,668,199 Freund, M. A. 5/26/87 Jennerjahn, P. H. Kusche, D. W. 4,684,351 Wantanabe, Yoshimi 8/4/87 Yamashita, Kazuya 4,790,134 Johnson, C. A. 12/13/88 4,795,383 Binversie, G. J. 1/3/89 Gerdes, R. W. Macier, J. E. 4,799,905 Broughton, G. L. 1/24/89 Rogers, D. E. Mate, D. W. 4,887,692 Outani, Hiroaki 12/19/89 Inoue, Seiji 4,917,640 Miles, C. C. Jr. 4/17/90 ______________________________________